President Obama signs the health care law on March 23, 2010. / Charles Dharapak, AP

by David Jackson, USA TODAY

by David Jackson, USA TODAY

President Obama hailed the fourth anniversary of his signature health care law on Sunday, and reminded supporters that a key deadline looms.

"Since I signed the Affordable Care Act into law, the share of Americans with insurance is up, and the growth of health care costs is down, to its slowest rate in fifty years," Obama said in a written statement.

Obama signed a health care bill into law on March 23, 2010. Now, the administration faces a March 31 sign-up deadline for coverage this year.

Republicans have also marked the fourth anniversary of the law, pledging to make it a major issue in congressional elections this year by arguing that it raises costs and cancels insurance policies.

In his statement, Obama cited aspects of the law that provide free preventive care with items like mammograms and contraception, and forbid insurance companies from placing dollar limits on insurance payouts.

The law also prevents discrimination against people with pre-existing medical conditions, and ends higher insurance costs for women, Obama said.

Republicans question whether enough young, healthy people will sign up for the plan in order to finance it.

In his statement on the four year anniversary of the health care law, Sen. Marco Rubio, R-Fla., said people have had to change doctors and specialty centers because of the new law.

"They've lost the health insurance they were happy with," Rubio said. "They're paying higher premiums, or a much higher deductible. Maybe it's cost them their job, maybe it's cost them hours at work, or maybe they're suffering from all of the above."

While Rubio and other Republicans vowed to repeal the law, Obama said his team would refine it.

"My administration will spend the fifth year of this law -- and beyond -- working to implement and improve on it," Obama said.